How to use lost foam casting to produce high-quality steel castings?
Lost foam casting has the advantages of simple process operation, low material consumption cost, smooth surface, high dimensional accuracy, and less environmental pollution. It has been vigorously promoted and applied in the late 1990s in my country, especially in iron castings. It has achieved good success. Experience, such as the application of gray iron parts in the box and shell. However, there is not much successful experience in the use of lost foam in steel castings, especially in low-carbon steel. The main reason is that lost foam castings are prone to carbon-increasing chemical composition deviation defects.
How to optimize and control the key processes in the production system to produce qualified steel castings with lower costs?
Prevent carbon increase
Lost foam steel castings are prone to carburization defects. The main reason for the carbon increase defects of steel castings is that the foam material contains carbon, and the foam burns and decomposes free carbon during pouring, and the carbon invades the molten steel.
After experimentation, it is found that the carbon increase has a certain regularity, that is, the surface of the casting increases carbon, but the core hardly increases carbon; there is no carbon increase near the inner gate, and the farther away from the inner gate, the more serious the carbon increase. Take the following corresponding measures to make the casting composition basically within the range of process requirements.
(1) Choosing foam materials with low carbon content is the key. At present, there are three main materials for lost foam casting: EPS, STMMA, and EPSMMA, and their carbon content decreases sequentially. Among them, EPS is characterized by large carbon content, but its gas generation is small, it is not easy to spray back during pouring, and its price is cheap, and it is widely used in iron castings and steel castings with low requirements.
EPSMMA is characterized by low carbon content, but large gas output, which is easy to cause backspray, and the material is expensive, and it is generally used more on low alloy steel. STMMA takes into account the advantages of both, has the advantages of less gas and low carbon content, and is the material of choice for the production of steel castings in lost foam.
(2) Taking advantage of the feature that the farther away from the inner gate, the more serious the carbon increase, the riser is set at the farthest distance from the inner gate or at the highest point of the casting, so that the molten steel with serious carbon increase pollution that enters the casting first enters the casting. In the mouth, the riser also plays a role of collecting slag and gas. The use of this process for steel castings can control the overall carbon content of the castings within the process requirements.
(3) The density of the lost pattern is very important. As long as the surface is smooth and the density is low, it can bring about the benefits of low carbon increase and less gas generation.
Prevent back spray
Back spray is a common phenomenon in lost foam casting. If the back spray is serious, it may endanger the personal safety of the caster. It must be paid attention to. In order to reduce the spray phenomenon, the following measures can be taken.
(1) The density of the foam model should be small. Under the premise of ensuring the quality of the foam surface and the strength of the model, the lighter the foam should be, the better, in order to reduce the amount of gas during pouring.
(2) The foam model must be dried before the paint is applied. Each batch of foam models of the steel castings should be sampled for the weight change during the drying process, and the baking weight change curve chart should be made. Only when the weight no longer changes. Paint.
(3) The pouring system, especially the sprue and cross runner, should not be coated with paint, so that the gas generated during pouring can be quickly and fully drawn away, and no coating is applied, which saves the consumption of paint.
(4) Cover a baffle plate on the top of the sprue cup, which can block the molten steel that is sprayed back up, so that it cannot splash out and endanger the on-site operators.
The main role of coatings in lost foam
(1) Avoid direct contact between molten steel and sand to ensure the surface quality of the casting;
(2) Improve the strength of the foam so that it does not deform during handling;
(3) The gas generated during pouring is exhausted and drawn away through the coating.
Paint brushing and paint drying
(l) It is best not to finish the paint at one time. Because the paint is very thick and easy to crack, it should be applied two to three layers. The overall thickness of the coating should be controlled at about 1mm.
The principle of controlling the coating thickness is that the coating should be as thin as possible on the premise that the casting does not stick to the sand, so that the gas can be discharged through the coating as soon as possible.
(2) Each layer of paint must be dried before the next round of painting or sculpting. If the coating is not completely dry, the molten steel will generate a lot of gas when it meets moisture during pouring, causing backspray. In addition, the coating If it is not completely dry, the paint has poor air permeability, and the gas cannot be discharged in time. The steel castings are likely to cause pores and slag holes in the castings.
When there are many castings in series in a box, because each model is vaporized at the same time, the vacuum is not enough, and it is easy to cause box collapse. To prevent box collapse, pay attention to:
(1) Maintain a sufficient and stable vacuum in the flask.
(2) Control the pouring temperature, and at the same time, the pouring speed should be as consistent as possible with the gasification speed of the model to prevent the cold barrier from being poured in too slowly and causing collapse of the box.
The most taboo of lost foam casting pouring operation is intermittent pouring, which easily causes cold barrier defects in castings, that is, the temperature of the molten metal poured first decreases, resulting in a cold barrier between the molten metal poured later.
In addition, the lost foam casting pouring system mostly uses a closed pouring system to maintain the pouring stability. In this regard, the form of the pouring cup is closely related to whether the pouring operation is stable. During pouring, the liquid level in the pouring cup should be kept stable to make the pouring dynamic pressure head stable.
Negative pressure is a necessary measure for black alloy lost foam casting. The role of negative pressure is an important measure to increase the strength and rigidity of the sand mold, and it is also the main measure to eliminate the model gasification products. The magnitude and holding time of the negative pressure are related to the casting material, the model cluster structure and the coating.
For coatings with good air permeability and coating thickness less than 1mm, the negative pressure on iron castings is generally 0.04～0.06MPa, and the upper limit is taken for steel castings. For aluminum castings, the negative pressure is generally controlled at 0.02～0.03MPa.
The holding time of the negative pressure depends on the structure of the model clusters. In the case of a large number of model clusters in each box, the holding time of the negative pressure can be appropriately extended. Generally, the negative pressure can be removed when the solidified crust on the surface of the casting reaches a certain thickness. For thicker coatings and poor air permeability, the negative pressure and holding time can be appropriately increased.